Earth-boring tools for forming wellbores in subterranean earth formations generally include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit. Similarly, roller cone earth-boring rotary drill bits may include cones that are mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which it is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit. In other words, earth-boring tools typically include a bit body to which cutting elements are attached.
The cutting elements used in such earth-boring tools often include so-called polycrystalline diamond compacts (PDCs), which employ a polycrystalline diamond material (PCD) as a shear-type cutter to drill subterranean formations. Conventional PDC cutting elements include a PCD cutting table and a substrate. The substrate conventionally comprises a metal material (e.g., a metal matrix composite such as cemented tungsten carbide), to enable robust coupling of the PDC cutting elements to a bit body. The cutting table typically includes randomly oriented, mutually bonded diamond (or, sometimes, cubic boron nitride (CBN) particles, in another, non-diamond superabrasive structure) that have been adhered to the substrate on which the cutting table is formed, under extremely high-temperature, high-pressure (HTHP) conditions. Catalyst material or binder material (e.g., cobalt binders) have been widely used to initiate bonding of diamond particles to one another and to the substrates, and catalyst material, usually in the form of cobalt, is often incorporated in the cemented tungsten carbide substrate.
Upon formation of a cutting table using a HTHP process, catalyst material may remain in interstitial spaces between the grains of diamond in the resulting PDC. The presence of the catalyst material in the cutting table may contribute to thermal damage in the cutting table when the cutting element is heated during use, due to friction at the contact point between the polycrystalline diamond cutting table of the cutting element and the formation.
PDC cutting elements in which the catalyst material remains in the PDC are generally thermally stable up to a temperature of about seven hundred and fifty degrees Celsius (750° C.), although internal stress within the cutting element may begin to develop at temperatures exceeding about three hundred and fifty degrees Celsius (350° C.). This internal stress is at least partially due to differences in the rates of thermal expansion between the cutting table and the cutting element substrate to which it is bonded. This differential in thermal expansion rates may result in relatively large compressive and tensile stresses at the interface between the cutting table and the substrate, and may cause the cutting table to delaminate from the substrate. At temperatures of about seven hundred and fifty degrees Celsius (750° C.) and above, stresses within the cutting table itself may increase significantly due to differences in the coefficients of thermal expansion of the diamond material and the catalyst material within the cutting table. For example, cobalt thermally expands significantly faster than diamond, which may cause cracks to form and propagate within the cutting table, eventually leading to deterioration of the cutting table and ineffectiveness of the cutting element.
Furthermore, at temperatures at or above about seven hundred and fifty degrees Celsius (750° C.), some of the diamond crystals within the PDC may react with the catalyst material causing the diamond crystals to undergo a chemical breakdown or back-conversion to another allotrope of carbon or another carbon-based material. For example, the diamond crystals may graphitize at the diamond crystal boundaries, which may substantially weaken the cutting table. In addition, at extremely high temperatures, in addition to graphite, some of the diamond crystals may be converted to carbon monoxide and carbon dioxide.
In order to reduce the problems associated with differential rates of thermal expansion and chemical breakdown of the diamond crystals in PDC cutting elements, so-called “thermally stable” PDCs (which are also known as thermally stable products or “TSPs”) have been developed. Such a thermally stable PDC may be formed by leaching the binder or catalyst material (e.g., cobalt) out from interstitial spaces between the inter-bonded diamond crystals in the cutting table using, for example, an acid or combination of acids. Thermally stable PDCs in which substantially all catalyst material has been leached out from the cutting table have been reported to be thermally stable up to temperatures of about twelve hundred degrees Celsius (1,200° C.). Some conventional TSPs, instead of being leached of catalyst, also incorporate silicon material in voids between the diamond particles.
However, problems with such PDC cutting elements including cutting tables formed from TSP include difficulties in achieving a good attachment of the cutting table to a supporting substrate due largely to the lack of the solvent catalyst material within the body of the cutting table. In addition, silicon-filled TSPs do not bond easily to a substrate. Further difficulties include providing adequate support of the cutting table on the substrate during drilling operations. The substrate and cutting table of a TSP cutting element are generally bonded using a material (e.g., a brazing alloy or other adhesive material) having a relatively lower hardness as compared to the hardness of the cutting table and substrate. TSPs, and particularly leached TSPs with open voids between the diamond particles, have proven to be undesirably fragile if not adequately supported against loading experienced during drilling. During a drilling operation, the PDC cutting elements are subjected to relatively high forces and stresses as the PDC cutting elements are dragged along a subterranean formation as a drill bit to which they are secured is rotated under weight-on-bit (WOB) in order to form a borehole. As the cutting table is dragged along the formation, the material bonding the cutting table to the substrate, having a relatively lower hardness and less stiffness than either of the bonded components of the cutting element may compress or otherwise deform in a non-uniform manner, subjecting the cutting table to tensile stresses, or combined tensile and compressive stresses (e.g., bending) during drilling operations. Such stresses on the substantially inelastic PCD material of the cutting table may lead to crumbling and cracking of the polycrystalline diamond structure and result in failure of the cutting element due to failure of the cutting table or the bond at the interface between the cutting table and substrate.